1. Field of the Invention
The present invention generally relates to a planar light source and a direct-vision display device using such planar light source suited for use in personal computers, computer monitors, video cameras, television receiver sets, car navigation systems, advertising billboards and so on.
2. Description of the Prior Art
A transmission type display device, particularly that represented by a liquid crystal panel and an advertising video billboard, is generally made up of a planar light source element (backlight source) for emitting light from one surface thereof and a display panel for presentation of images. In this transmission type display device, controlling the transmissivity of light through the display panel allows in images such as letters and/or pictures presented on the display panel to be viewed by those watching the display device. The backlight source is nowadays available in various types including the type in which while a halogen lamp, a reflecting plate and a lens are employed in combination the pattern of distribution of luminance of light emerging outwardly from the display device is controlled; the type in which a tubular fluorescent lamp is positioned adjacent one end face of a light guide member so that light from the tubular fluorescent lamp can emerge outwardly from one surface of the light guide member lying orthogonal to such one end face thereof; and the type (in-laid type) in which a tubular fluorescent lamp is positioned inside a light guide member. The backlight source utilizing a halogen lamp is mainly used in a liquid crystal video projector that generally requires a kind requiring a high luminance. On the other hand, the backlight source employing a light guide member is in most cases used in a direct vision liquid crystal television monitor and a personal computer display because the light guide member makes it possible to reduce the thickness of the backlight source. The backlight source of the in-laid type is simple in structure and is, therefore, mostly used in relatively large lighting billboards.
It is well known that low power consumption and light output of a high luminance are important requirements that the backlight source used particularly in liquid crystal television receiver sets and note-size personal computers should meet. Although the high luminance light output can be readily available if the number of light sources such as cold cathode fluorescent lamps used is increased, the use of the increased number of the light sources is not practical as it leads to increase of the electric power consumption.
Accordingly, a planar light source device including light sources, a light guide member and an array of microprisms such as shown in FIG. 18 has hitherto been employed and suggested in, for example, U.S. Pat. No. 5,396,350. According to this US patent, since the microprisms are formed on a plane, the array of the microprisms distributed over such plane is susceptible to variation in pattern of distribution of angles of emergence of light from the planar light source device and, therefore, no sufficient frontal luminance characteristic can be secured virtually.
Where in the planar light source device an array of microlenslets is disposed on one surface of the light guide member remote from the microprism array, a sufficient frontal luminance characteristic can be secured with no substantial variation occurring in pattern of distribution of angles of emergence of light from the planar light source device. However, in this type of the planar light source device, the positional relationship between the microprisms and the microlenslets must be set up accurately. While accurate positioning of the microlenslets relative to the microprisms appears to be possible if the microlenslet array and the microprism array are spaced a substantial distance from each other, a relatively small spacing between the microlenslet array and the microprism array makes it extremely difficult to achieve an accurate alignment between the microlenslets and the microprisms, bringing about a cause of reduction in productivity and, hence, increase of the manufacturing cost.
As discussed above, the use of the planar array of the microprisms makes it difficult to eventually provide the planar light source device of a type in which utilization of light from the light source is maximized while exhibiting a relatively high frontal luminance characteristic.
Where in the microprism array of the structure shown in FIG. 18 the light source, for example, a tubular lamp is disposed in the vicinity of only one end face of the light guide member, only one of facets of each of the microprisms is utilized in dealing with the incident light from the light source while the facet of each microprisms opposite to such one of the facets is not utilized in this sense and, therefore, no region of each microprism adjacent the non-utilized facet is effectively and efficiently utilized, resulting in reduction in efficiency of utilization of light. Hence, the use of the light source adjacent only one end face of the light guide member is ineffective to provide the planar light source device capable of exhibiting a high luminance.
Even in the planar light source device of the structure shown in FIG. 18, depending on a combination of refractive indexes of materials used to form the respective component parts, light emerging outwardly from the planar light source device in a direction slantwise relative to a light exit surface thereof having diverged from a frontal direction normal to the plane of the light exit surface cannot be suppressed to a low illuminance and, hence, the efficiency of utilization of light is relatively low, failing to provide the planar light source device capable of exhibiting a high luminance.
On the other hand, the Japanese Laid-open Patent Publication No. 8-221013, for example, discloses a backlighting device of a structure comprising a first planar light guide member for directing light, incident thereon from a light source positioned adjacent one end of such first planar light guide member, so as to provide a plane-oriented light traveling in a direction perpendicular to the first planar light guide member, and a second planar light guide member having a multiplicity of surface-curved projections formed on one surface thereof adjacent the first planar light guide member.
This known backlighting device has been found having a problem. Specifically, since the angle of inclination of the curved surface of each of the projections formed in the second planar light guide member, particularly that of the curved surface adjacent the base of each projection, is so large (for example, each projection being of an elliptical shape in which the ratio between the major axis relative to the minor axis is 1:2) that light incident on the second planar light guide member from a light source tends to be reflected backwards towards the light source in a relatively large proportion. Because of the relatively large proportion of light reflected backwards towards the light source, the intensity of the plane-oriented light traveling in the frontal direction tends to be lowered.
Also, in the backlighting device of the type employing the above described multiplicity of surface-curved projections, it has been suggested to provide the light guide member with a light controlling sheet for facilitating emergence of the outgoing light. Since in this structure, total reflection of light is utilized and a loss of light is therefore minimal, allowing the display device to exhibit a high luminance. However, in this backlighting device, tips of the projections on the light controlling sheet have to be optically bonded to the light guide member. To achieve this bonding, while the use may be contemplated of a highly transparent adhesive agent, a highly transparent ultraviolet curable resin or a highly transparent thermosetting resin, a point or line bonding is generally employed to bond the tips of the projections on the light controlling sheet to the light guide member and, therefore, a problem has been found in that the bonding strength is low. Specifically, in the event of change in environment such as change in temperature and/or relative humidity, the difference in thermal expansion coefficient and/or hygroscopic expansion coefficient between the light controlling sheet and the light guide member leads to generation of a relatively large shearing force in the vicinity of the perimeter of the planar light source device, resulting in separation of the light controlling sheet from the light guide member.
In view of the foregoing, the present invention has been devised to provide an improved planar light source device capable of maximizing utilization of light and exhibiting a high luminance in a frontal direction, i.e., in a direction forwardly of and normal to the light exit surface thereof.
Another important object of the present invention is to provide an improved display device utilizing the planar light source device of the type referred to above and capable of exhibiting a high luminance.
In order to accomplish these objects of the present invention, there is, in accordance with one aspect of the invention, provided a planar light source device which includes a light source, a reflector shade, a light guide member having a light exit surface and also having at least one side face positioned adjacent the light source for receiving light that is emitted from the light source and reflected by the reflector shade, and a light output plate having first and second surfaces opposite to each other and positioned in face-to-face relation with the light guide member with the first surface positioned adjacent the light exit surface of the light guide member. The first surface is formed with a multiplicity of projections having respective tips held in tight contact with the light exit surface of the light guide member, at least a portion of each of the projections having a curved face. Of a function descriptive of a sectional shape of each of the projections that is parallel to a direction of travel of light and also a direction normal to the light output plate, the maximum value of the absolute value of a linear differential of a portion representative of a facet counter to the light source is chosen to be within the range of 1 to 3.
The term xe2x80x9cdirection of travel of lightxe2x80x9d referred to hereinbefore and hereinafter is intended to means a direction parallel to the light exit surface of the light guide member and orthogonal to a light emitting surface of the light source. The light emitting surface of the light source referred to above means, where the light source is in the form of, for example a tubular fluorescent lamp as shown in FIG. 1, an outer peripheral surface thereof.
According to the structure described above, when of a function descriptive of a sectional shape of each of the projections that is parallel to a direction of travel of light and also a direction normal to the light output plate, the maximum value of the absolute value of a linear differential of a portion representative of a facet counter to the light source is chosen to be within the range of 1 to 3, the luminance in the frontal direction can be increased and, therefore, the planar light source device exhibiting a high efficiency of utilization of light and a high luminance in the frontal direction can be obtained.
Preferably, of the function descriptive of the sectional shape of each of the projections a quadric differential value of that portion representative of the facet counter to the light source is chosen to be within the range of xe2x88x9210to 20 [1/ unitary length], where the unitary length (a. u.) is represented by a length of such each portion of projection as measured in a direction parallel to the direction of travel of light.
Also preferably, the fimction descriptive of the sectional shape of each of the projections has a point of inflection.
If the planar light source device of the present invention is combined with the transmissive type display element, for example, a liquid crystal display element, a printed film and a molded member having a light scattering capability, a display device capable of exhibiting a high luminance in the frontal direction can be obtained.
According to another aspect of the present invention, the planar light source device is featured in that each of the projections has an axis inclined relative to the direction normal to the light output plate and forms an acute angle relative to the direction of travel of light.
The axis of each projection is represented by an Y-axis of the system of coordinates that is set up so that the function descriptive of the sectional shape of each projection taken in a direction parallel to the direction of travel of light and also a direction normal to the light output plate represents an approximately even function.
According to this preferred design, the proportion of the facets of the projections of the light output plate, which confront the light incident end face and are not utilized can advantageously be reduced so that the number of the projections that can be formed on the light output plate can be increased to increase the luminance.
According to a further aspect of the present invention, the light guide member and the tips of the projections of the light output plate are optically coupled with each other through at least one optically transparent layer.
This design is particularly effective in that since the intensity of light emerging outwardly in a direction slantwise, i.e., diverting from the frontal direction can be lowered, it is possible to provide the planar light source device having a high efficiency of utilization of light and a high luminance.
In one preferred embodiment, the projections of the light output plate has a refractive index that is greater than the refractive index of the light guide member and the refractive index of the optically transparent layer is greater than the refractive index of the light guide member.
In addition, the optically transparent layer positioned between the light guide member and the light output plate preferably has a refractive index that is smaller than the refractive index of the projections of the light output plate.
According to a still further aspect of the present invention, the planar light source device of the present invention is featured in that the optically transparent layer includes at least an intermediate film and a bond layer disposed on each of opposite surfaces of the intermediate film and the ratio of a hygroscopic expansion coefficient of the intermediate film relative to that of the light output plate is chosen to be within the range of 0.1 to 10 and/or the radio of a thermal expansion coefficient of the intermediate film relative to that of the light output plate is chosen to be within the range of 0.5 to 2.
According to this design, since the shearing stress resulting from change in environment can be set up between the light guide member and the intermediate film, the shearing stress which will develop between the projections and the intermediate film for emergence of light can be relieved, making it difficult for the light output plate to separate. Accordingly, the planar light source device robust against change in environment can be obtained.
The present invention also provides a method of making the planar light source device of the structure described above. During the manufacture of the planar light source device, the tips of the projections of the light output plate and the intermediate film are bonded together by means of a bonding agent and the intermediate film and the light guide member are also bonded together by means of a bonding agent. It is to be noted that the term xe2x80x9cbonding agentxe2x80x9d referred to above and hereinafter is intended to encompass not only a bonding material which undergoes hardening to exhibit a bonding strength, but also an adhesive agent that does not harden.
Preferably, the step of bonding the tips of the projections of the light output plate and the intermediate film together includes a substep of contacting them by means of a semi-hardened bonding agent and a substep of subsequently hardening the semi-hardened bonding agent completely.